Interest in single-site (metallocene and non-metallocene) catalysts continues to grow rapidly in the polyolefin industry. These catalysts are more reactive than Ziegler-Natta catalysts, and they produce polymers with improved physical properties. The improved properties include narrow molecular weight distribution, reduced low molecular weight extractables, enhanced incorporation of .alpha.-olefin comonomers, lower polymer density, controlled content and distribution of long-chain branching, and modified melt rheology and relaxation characteristics.
Traditional metallocenes commonly include one or more cyclopentadienyl groups, but many other ligands have been used. Putting substituents on the cyclopentadienyl ring, for example, changes the geometry and electronic character of the active site. Thus, a catalyst structure can be fine-tuned to give polymers with desirable properties. Other known single-site catalysts replace cyclopentadienyl groups with one or more heteroatomic ring ligands such as boraaryl (see, e.g., U.S. Pat. No. 5,554,775), pyrrolyl, indolyl, (U.S. Pat. No. 5,539,124), or azaborolinyl groups (U.S. Pat. No. 5,902,866).
Single-site catalysts typically feature at least one polymerization-stable, anionic ligand that is purely aromatic, as in a cyclopentadienyl system. All five carbons in the planar cyclopentadienyl ring participate in bonding to the metal in .eta.-5 fashion. The cyclopentadienyl anion functions as a 6.pi.-electron donor. Similar bonding apparently occurs with heteroatomic ligands such as boratabenzenyl or azaborolinyl.
In contrast, olefin polymerization catalysts that contain "homoaromatic" anions are not known. "Homoaromatic" refers to systems in which a stabilized, conjugated ring system is formed by bypassing a saturated atom. (See F. Carey and R. Sundberg, Advanced Organic Chemistry, 3.sup.rd Ed., Part A, 518-520 (1990).) The observation of .sup.1 H NMR aromatic ring currents helped to identify the homotropilium cation (see R. Childs, Acc. Chem. Res. (1984) 17, 347). Unexpectedly rapid deprotonation of bicyclo[3.2.1]octa-2,6-diene demonstrated generation of a bishomoaromatic cyclopentadienide anion (see J. Brown and J. Occolowitz, Chem. Commun. (1965) 376): ##STR1##
This is a "bishomoaromatic" system because two saturated carbons (at the bridgeheads) are bypassed to give the conjugated, stabilized anion. S. Winstein and coworkers confirmed the presence of the bishomoaromatic anion by .sup.1 H NMR (see J. Am. Chem. Soc. 89 (1967) 3656). L. Paquette summarizes a wealth of information about homoaromaticity in a thorough review article (Angew. Chem. Int. Ed. Engl. 17 (1978) 106).
In spite of the availability of synthetic routes to homoaromatic anions, their use as ligands for metallocene or single-site catalysts for olefin polymerization has not been suggested. On the other hand, the ease with which a host of interesting homoaromatic ligands can be prepared suggests that catalysts with advantages such as higher activity and better control over polyolefin properties are within reach. Ideally, these catalysts would avoid the all-too-common, multi-step syntheses from expensive, hard-to-handle starting materials and reagents.